The invention relates to an optical record carrier comprising a recording layer having substantially parallel tracks for recording user information in a pattern of optically detectable marks, neighbouring tracks being separated by an edge, subsequent edges having subsequent ordinal numbers, each edge having a edge, subsequent edges having subsequent ordinal numbers, each having a transverse position with a varying deviation from an average transverse position, the deviation of even-numbered edges having alternately a first and a second periodic modulation representing track-dependent control information.
The invention also relates to a method of scanning such a record carrier and an apparatus for scanning it.
In general, a track is a line on the record carrier to be followed by a scanning device and having a length of the order of a characteristic dimension of the record carrier. A track on a rectangular record carrier has a length smaller than the length or width of the record carrier. A track on a disc-shaped record carrier is a 360xc2x0 turn of a continuous spiral line or a circular line on the disc. The tracks are separated by edges. An edge is a change in the value of an optically detectable parameter when going from one track to a neighbouring track. For example, the reflectivity of the recording layer may change between tracks. The edge may be a groove or ridge of a relatively small width in between neighbouring tracks. When neighbouring tracks are at different heights with respect to the plane of the recording layer, i.e. the tracks are located on lands and grooves, the edges are formed by the groove walls between the tracks. When the tracks are parts of a spiral the tracks on land and the tracks in groove may each form one continuous spiral over the recordable area of the record carrier.
When writing user information on a record carrier by means of a scanning radiation spot, it is in general desirable to know the position of the radiation spot along a track on the record carrier. Since for this purpose user information is not available on a virgin recordable record carrier, the position information may be retrieved from the edges if the position information is encoded in a modulation of the transverse position of the edges of the record carrier. In general, the edges may represent control information in which the position information is comprised.
A record carrier having information stored in the transverse position of the edges is known from the Japanese patent application no. 06338066. The record carrier described therein comprises alternating first and second grooves in a substrate. User information may be recorded both in the grooves and on the lands between the grooves. Both edges of a groove are modulated by a transverse wobble of the centre line of the groove. The first grooves are frequency modulated at a relatively low frequency, the second grooves are frequency modulated at a relatively high frequency. When scanning a track located in a groove, the scanning spot is modulated only by the modulation of the groove, and the scanning device can read information encoded in the modulation by choosing a low-frequency decoder for a first groove and a high-frequency decoder for a second groove. When scanning a track on the land portion in between two grooves, the spot is modulated by the modulation of the edges of both neighbouring grooves. The scanning device can then discriminate between signals from the first and second groove by switching between the low- and high-frequency decoder to read the position information of the neighbouring first and second groove.
To achieve an accurate positioning of the scanning spot on the record carrier, the density of the control information in the grooves should be made as high as possible. However, the density is limited by crosstalk of the groove modulation on the signal representing the user information. In the known record carrier the frequency of the modulation of the second groove may be chosen near the limit imposed by the crosstalk. The frequency of the modulation of the first grooves must be substantially lower than the frequency of the second grooves to be able to separate the two frequencies in the scanning device and thereby create a low crosstalk between both frequencies. The substantially lower frequency results in a lower information density. Hence, the first grooves have a relatively low position information density.
It is an object of the invention to provide a record carrier and scanning method having a high control information density.
In accordance with an aspect of the invention, the record carrier as described in the opening paragraph is characterized in that the first and second periodic modulations have a predetermined phase relation and in that the deviation of an odd-numbered edge is in the same direction as the deviation of one of its neighbouring edges at the position along the track where the deviation of its other neighbouring edge is substantially equal to a predetermined value.
Whereas in the prior art both edges of a groove are modulated identically, the edges of a track in the record carrier according to the invention are modulated differently. A track according to the invention is, in the transverse direction, bound by an even-numbered edge and an odd-numbered edge. The transverse position of the even-numbered edge is encoded with control information. When scanning the track by means of a scanning spot, the control information is retrieved by sampling the transverse positions of both edges at regular positions along the track. The sample positions are determined with respect to the previous or next even-numbered edge which neighbours the odd-numbered edge of the track being scanned. The sampling positions correspond to the positions along the track where the deviation of the previous or next even-numbered edge is substantially equal to a predetermined value. At these positions, the odd-numbered edge has a transverse deviation in the same direction as the deviation of the even-numbered edge of the track. The equal direction of the deviations of both edges of the track enhance the signal derived from the edge positions, thereby improving the quality of the reading of the control information. The sample positions for neighbouring tracks are shifted in the longitudinal direction, because of the phase shift between the modulations of the transverse edge positions of the even-numbered edges. Therefore, the odd-numbered edge located between two even-numbered edges can have a deviation in the appropriate direction for the sample positions of both tracks it separates. The specific variation of the transverse position of the edges allows a high control information density, independent of the track. Moreover, the strength of a read signal obtained from the deviations may be made equal for all tracks of the record carrier.
To simplify the detection of the transverse positions, the predetermined value of the deviation is preferably equal to zero, i.e. the deviation of the edge is equal to the average deviation of the edge.
An odd-numbered edge has preferably a modulation which is proportional to the sum of the modulations of both neighbouring even-numbered edges.
The modulation of the edge position must have a high information density for accurate position information on the one hand and not too high a frequency content to avoid cross talk on the other hand. A sinusoidal modulation of the edge is a preferred compromise between these two requirements.
The phase shift between the modulations of neighbouring even-numbered edges is preferably substantially equal to 90xc2x0. As a consequence, if the modulation of one of the edges is a sine wave, the modulation of the other edge is a cosine wave. When a sample is taken during the scanning of a track at a first position of 90xc2x0, the deviation of the cosine modulated edge will be zero, and the detector signal directly indicates the value of the sine-modulated edge. When, likewise, a sample is taken at a second position of 0xc2x0, the deviation of the sine-modulated edge will be zero and the detector signal will indicate the deviation of the modulation of the cosine-modulated edge. As a result, the cross-talk between even-numbered edges is reduced.
In a preferred embodiment of the record carrier the control information is encoded in the edge positions by 180xc2x0 phase-shift keying. Since this type of encoding does not affect the positions where the edge deviation has a zero value, it can be combined suitably with the phase-shifted modulations of the invention.
Neighbouring tracks are preferably arranged at different heights with respect to the plane of the recording layer and the edges are walls between neighbouring tracks. The subsequent tracks will form a land-groove structure in which a track in a groove adjoins a track on land. One wall of a groove, the even-numbered edge, is position-modulated with control information. The other wall of the groove is modulated such that, at the sample positions, it enhances the read signal derived from the modulation of the even-numbered edge when scanning the groove. When scanning a land, one edge is similarly encoded with control information and the other edge enhances the signal of the first edge.
A proper definition of the positions along a track for taking samples can be obtained when the edge deviations are provided with clock marks. In that case the sampling positions can be defined with respect to the clock marks.
The modulation may contain parts representing position information, such as address information, and parts not representing position information, such as clock marks. The clock marks may have fast rising and falling edges, used in the detection of the clock marks. The clock marks in adjacent edges are preferably aligned in a direction transverse to the tracks. To reduce interference with the reading of user information, adjacent clock marks have preferably the same phase.
In order to prevent that an edge modulation representing control information is detected as a clock mark, the modulation representing control information has preferably a finite derivative with respect to the position along the track. When the track modulation is sinusoidal, this can be realized by using parts of a sine wave beginning or ending at 0xc2x0 or 180xc2x0 and parts of a cosine wave beginning or ending at 90xc2x0 or 270xc2x0. If necessary, the modulation pattern may be completed to have a predetermined, fixed length by adding track parts having no variation at the beginning or end of a sinusoidal variation.
In a accordance with a further aspect of the invention, a method of scanning a record carrier is provided as described in the claims.
In accordance with a still further aspect of the invention, an apparatus is provided for scanning an optical record carrier according to the invention as described in the claims.